Modern electrical therapeutic and diagnostic devices for the heart, such as pacemakers, cardiovertors, and defibrillators, for example, require a reliable electrical connection between the device and a region of the heart. Typically, a medical electrical "lead" is used for the desired electrical connection.
One type of commonly used implantable lead is a transvenous lead. Transvenous leads are positioned through the venous system to attach or electrically connect at their distal end to the heart. At their proximal end, they are connected to typically an implantable pulse generator. Such leads normally took the form of a long, generally straight, flexible, insulated conductor. Among the many advantages of a transvenous lead is that it permits an electrical contact with the heart without physically exposing the heart itself, i.e., major thoracic surgery is not required.
The specific design of a transvenous lead used is often varied depending upon the region of the heart to which it is to be connected. For example, U.S. Pat. No. 4,402,330 of Lindemans discloses a body implantable lead in which the lead body has a J-curve and the distal electrode has a permanent bend. In such a manner, the lead is configured to electrically connect to the right atrium.
While such a lead has been found acceptable for electrically connecting and thus pacing the right atrium, the need exists for a transvenous medical electrical lead which may provide an electrical connection to the left atrium. Of course the left atrium cannot, at present, be transvenously accessed with a lead for chronic implantation due to the direction of blood flow and the present limitations of materials. To be precise, blood flows through the right side of the heart (atrium and ventricle), through the lungs, through the left side of the heart (atrium and ventricle) and then through the rest of the body, including the brain, before returning again to the right side of the heart. Implanted objects, however, often cause minor blood clots and thrombus to form in the blood. These may, on occasion, dislodge and be released into the bloodstream. Because the blood circulates directly from the left atrium and ventricle to the brain, any clots, however minor, could have serious consequences if they were to reach the brain, e.g. a stroke. In contrast, any clots released from an object implanted in the right side of the heart would simply travel to the lungs, where they would lodge without any serious risk. Thus at present, chronic transvenous leads may not be safely implanted within the left side of the heart.
In spite of the difficulties, there remains a great need to be able to electrically stimulate or sense or both the left side of the heart. The most obvious reason is the left side of the heart accounts for the majority of the heart's hemodynamic output. For example, the left ventricle has a greater wall thickness (10-20 mm as compared to 1-5 mm) than the right side. This, of course, is reasonable given that the left side of the heart must pump blood throughout the body while the right side only pumps blood through the lungs.
Because the left side is relatively more important for hemodynamic output, not surprisingly various pathologies may be better treated through stimulation on the left side of the heart. For example, in patients with dilated cardiomyopathy, electrical stimulation of both the right side and the left side of the heart has been shown to be of major importance to improve the patient's well-being and manage heart failure. See, for example, Cazeau et al., "Four Chamber Pacing in Dilated Cardiomyopathy," PACE, Nov. 1994, pgs. 1974-79. See also Brecker and Fontainem, St. et al., "Effects Of Dual Chamber Pacing With Short Atrioventricular Delay In Dilated Cardiomyopathy," Lancet Nov. 1992 Vol. 340 p1308-1312; Xiao HB et al., "Effect Of Left Bundle Branch Block On Diastolic Function In Dilated Cardiomyopathy," Br. Heart J 1991, 66(6) p 443-447; and Fontaine G et al, "Electrophysiology Of Pseudofunction," CI.Meere (ed.) Cardiac pacing, state of the art 1979, Pacesymp, 1979 Montreal.
Many times the left side is to be electrically stimulated or sensed, the right atrium is also to be electrically stimulated or sensed. At present such multi-chamber pacing is accomplished using two separate leads, one for the right atrium and one for the left. Such a multi-lead system for left and right side therapy, however, has several drawbacks.
First, the two separate lead bodies may rub, over time leading to a breach in the insulative sheath and lead failure. Second, because two leads are required the cost of the entire system is correspondingly higher. In addition, the implant time, because two leads must be implanted, is also greater than if one lead could be used. Moreover, the two leads have a greater surface area exposed to the blood as compared to a single lead system. Although current surfaces of leads are highly successful, it is nonetheless a goal of all lead designs to minimize the surface area of the lead exposed to the blood to thereby decrease the possibility of any adverse blood-device reactions, e.g. thrombus.
Besides the problems inherent in a multi-lead system, there are still further difficulties with implanting and fixing a lead within the coronary sinus. Unlike a heart chamber where the fibrotic tissue response is used to assist lead fixation, no such fibrotic response can be expected to occur within the coronary sinus. As such no fibrotic tissue response is available to assist in lead fixation. Moreover, blood flow through the coronary sinus tends to sweep out any leads implanted therein. Finally the coronary sinus also presents the unique challenge that any lead must be fixed without the assistance of trabeculae, those structures in the right atrium and ventricle used for maintaining lead position.
It is thus an object of the present invention to provide a medical electrical lead which may be used to electrically access both the right atrium as well as either or both of the left chambers of the heart.
A still further object of the present invention is to provide such a medical electrical lead which may be used to electrically access both the right atrium as well as either or both of the left chambers of the heart and may be reliably fixed within the coronary sinus.